Just needed a restart of EF to get it to work.
- stop dropbox
- move .dropbox.cache away from EF library
- delete reference to .dropbox.cache in EF
- issue:
defaults write com.c-command.EagleFiler UnscannedExtensions -array cache
- quit EF
- move .dropbox.cache back
- relaunch EF
Seems to have worked!